The compound estradiol-3-N-bis(.beta.-chloroethyl)-carbamate-17.beta.-dihydrogen phosphate has been found to be very useful as an antitumour agent in the clinic (see e.g. Cancer Chemotherapy Reports Part 1, vol. 59, No. 1, 1975). This compound is designated as Estramustine Phosphate herein, this being its generic name and the established INN name. (Recommended International Non-Proprietary Name.)
The compound is presently used in the clinic either as its N-methylglucamine salt or as its disodium salt depending on the way of administration (intravenously or orally).
A method for the synthesis of antitumour compounds, having a structure derived from estradiol-17-dihydrogen phosphate, is described and claimed in British Pat. No. 1,016,959. In Example 15 of its specification, the preparation of Estramustine Phosphate is described. According to this example, said compound has been isolated by pouring its crude pyridinum salt into an excess of hydrochloric acid. The precipitate obtained is collected and washed with 0.1 N hydrochloric acid and water. The compound obtained is said to melt with decomposition at about 155.degree. C, to have an [.alpha.].sub.D.sup.20.degree. C = +30.0.degree. (c = 1.0 in dioxane), and to be soluble in an aqueous solution of alkali.
It has now been found that the compound obtained according to said example is not pure and, inter alia, always contains at least 3-4% and mostly more than 5% of pyridine as an impurity. This content of pyridine is only slightly reduced by repeated reprecipitations of the phosphate ester from alkaline solutions with hydrochloric acid -- operations which also are very difficult to perform as the free acid under such conditions is precipitated in a jelly-like form which makes it practically impossible to collect and wash.
Estramustine Phosphate and salts thereof may also contain degradation products such as estradiol-17-dihydrogen phosphate and its corresponding salts.
As far as the salts of Estramustine Phosphate are concerned, it has been found that said impurities are practically impossible to remove by recrystallization. To purify such an impure free acid, repeated recrystallizations from mixtures of ethanol-hexane can be used, but result in a very low yield of the pure acid.
It has now been found that impure Estramustine Phosphate can be purified by conversion to a molecular complex as by crystallization from a medium containing an alcohol having at least three carbon atoms, whereby said molecular complex formed is an addition compound between the pure acid and the alcohol used.
Although no theoretical upper limit for the number of carbon atoms in the alcohol exists, in general a maximum of eight carbon atoms represents the upper limit imposed by certain practical limitations such as melting points and boiling points.
It has also been found that impure salts of Estramustine Phosphate can be purified by converting said salts to said pure complexes. These complexes are stable compounds which can subsequently be dissolved in a solvent other than the type of solvent employed in its preparation with resulting precipitation of pure Estramustine Phosphate itself, or they may be dissolved in a suitable solvent in the presence of a suitable source of sodium to precipitate out a pure Estramustine Phosphate sodium salt. The source of the starting impure Estramustine Phosphate may be the free acid itself or any salt thereof which can readily be converted to the starting Estramustine Phosphate free acid by acidification.
Pure Estramustine Phosphate and its pure salts are obtained in high yields from said complexes by crystallization from suitable solvents or mixtures thereof.